Low-pollution alcohol-blended gasoline receives attention as fuel of vehicles. An appropriate air-fuel ratio of such a blended gasoline is different from that of fuel including only gasoline. In order to control the air-fuel ratio of the blended gasoline to have an appropriate value, it is important to measure an alcohol content in the blended gasoline, that is, an alcohol concentration.
It is preferable that a physical constant having a relatively-high change ratio is used so as to measure the alcohol concentration with a high degree of accuracy. Conventionally, a method for detecting a change of a relative permittivity is disclosed. The relative permittivity can be obtained from a change of capacitance. For example, JP-U-1-163862 discloses a fuel sensor that measures the capacitance by using a pair of electrodes arranged to be opposed to each other.
However, in the above method, the pair of electrodes arranged to be opposed to each other has a cantilever structure, and thereby abnormal noise may be generated due to fuel pulsation or the like. The generation of the abnormal noise may bring a feeling of strangeness to a driver.
Further, if a relative positional relationship of the electrodes is changed due to the fuel pulsation, detection accuracy of the capacitance may be reduced.
Although the detection of the capacitance of alcohol-blended gasoline is described as an example, the above-described problem also may occur in the case of detecting a fuel property by using a pair of electrodes. An example of “the fuel property” includes capacitance or a relative permittivity that can be detected by the pair of electrodes.